We don’t really have any infrastructure in place at the moment (like if-up.d) to make it easy to add static routes, as routes need to be re-added each time an interface comes up and connman doesn’t have direct support for configuring the routing table, other than the default route.
But you have another more serious hurdle - connman will not automatically connect two networks at once. So whilst you could manually connect Wifi while Ethernet is already connected, next time you reboot Ethernet will take precedence and Wifi will not automatically connect.
So what you are trying to do is currently not possible.